The El Camino was based on 64 promo tooling, changed to make the 64 3 in 1 annual kit, then up dated to the 65. So this tooling has been around for 43+ years! Traditionally the annual kits were based on the promo tooling and the "Big Three" usually had sheet metal changes once a year so the annual kits weren't as detailed as the "Trophy Series" kits that didn't require yearly changes.
If you're not to far along on the El Camino look for Revell's 66 Chevelle wagon, for a detailed chassis. Revell's 65 Chevelle SS 396 and 66 GTO, AMT's 66 442 and Lindberg's 67 442 have prototypically correct chassis' for the El Camino. The wheel base may have to be altered somewhat to make them fit. Also Model Car Garage offers a photo-etched detail set that has the emblems, grille and other parts that can be adapted to the El Camino.
It looks like you're doing well on all three!